The present invention relates to a method and to an apparatus for detecting the presence of water in a riser tensioning buoy in marine environment installations.
FIG. 1 shows an example of a known riser 10, implemented in a marine environment oil extraction installation. The riser 10 is typically employed for carrying, in a substantially vertical direction, oil products derived from one or more wells drilled in the seabed 12 to a surface structure, for example up to a floating base 18 for production, storage and loading (or FPSO “Floating production storage and offloading”). Hydrocarbons, once extracted from the wells, first pass through a network of ducts 14 which extend on the surface of the seabed between the wells and the lower end of the riser 10. Each duct connects one or more wells to the lower end of the riser 10.
The set of ducts are grouped together in the riser 10 and discharge at the upper end of the riser. Flexible pipes 16 running from the upper end of the riser 10 link the ducts 14 to the surface structure 18 where the hydrocarbons produced can be stored. The riser 10 and the pipes 16 can also be used in the opposite direction in order to, for example, convey water or gas from the floating base to the bottom.
The riser 10 may typically have a length of the order of a few kilometers. As a consequence, regardless of the material used to make the riser 10, the latter has a tendency to buckle. Moreover, this tendency can be accentuated by ocean currents. Also, in order to maintain riser 10 in the vertical position, it is known to associate it with a tensioning buoy 20, said riser being fastened to the seabed, at its lower end by means of an anchor 22.
Tensioning buoy 20 is generally a metal structure containing air in order to establish, thanks to buoyancy forces, an upward tension on riser 10, ensuring retention thereof in a vertical position and limiting buckling. FIGS. 2 and 3 show a known type of tensioning buoy. Tensioning buoy 20 is comprised of flat-bottomed 32 cylindrical chambers 24 fixed to each other and communicating with each other through valves 26. Each cylindrical chamber 24 has reinforcing members 28 on its upper and lower surfaces. In addition, a tube 30, designed to be rendered integral with the riser, passes through all the chambers 24 in order to distribute the forces exerted by the riser on tensioning buoy 20.
In a variant shown in FIG. 4, the tensioning buoy 40 includes cylindrical chambers 44 with a rounded bottom 42 attached to each other and communicating with each other through valves 46. The purpose of the rounded bottoms 42 is to improve mechanical strength compared to flat-bottomed chambers.
Tensioning buoys which remain permanently in a marine environment are subject to corrosion that can cause leaks in their chambers and thus render a tensioning buoy inoperative. Tensioning buoys are regularly inspected by a robot submarine to determine whether they are leaking and/or are partially filled with water. To do this, it is known to place a radioactive source on one side of the tensioning buoy and a radiation sensor opposite the source, at the other side of the buoy. Since water acts as a screen to radiation, the presence of water is determined when the level of radiation sensed falls below a predetermined threshold.
Although relatively reliable, this known method requires the use of a radioactive source, which presents obvious security and safety hazards. In addition, it is difficult to obtain such a radioactive source and import it to the place of use. Finally, depending on the geometry of the tensioning buoy, it may be difficult to determine the above threshold.
Furthermore, it is known to monitor the integrity of the structure of an offshore platform made of cylindrical steel tubes, by making use of an ultrasound transducer (or sensor/transmitter) placed on one of said tubes. More specifically, the transducer, placed against a wall of the tube, transmits ultrasound waves that propagate extremely well in water but almost not at all in air, and then measures echoes of the waves reflected by the wall of said tube opposite to the wall against which the transducer is placed.
This method is effective in the case of a structure of an offshore platform comprising cylindrical tubes of limited size but it is difficult to transpose the method to tensioning buoys. As FIGS. 2 and 3 show, the tensioning buoy 20 does indeed have a central tube 30 which tends to spread an incident ultrasound wave in diverging directions. It then becomes extremely difficult if not impossible to measure the echo of the ultrasound waves reflected from this tube and/or by the opposite wall. Moreover, given the diameters of buoys of this type, parallelism tolerances which are necessary for proper return of the ultrasound wave to the sensor are hardly compatible with conventional tolerances applying to such products from the boiler-making industry.
Furthermore, the two known processes described above are difficult to apply, or even inapplicable, for detecting water in a buoy having:                a diameter greater than four meters,        round-bottomed chambers.        
There is therefore a real need for a method and apparatus which are easy to implement, making it possible to accurately determine the presence or absence of water in a tensioning buoy. To this end, the present invention provides a method for detecting water in a riser tensioning buoy in installations in a marine environment, said method being characterized in that it comprises the successive steps of:                defining a representative standard echo        transmitting said ultrasound signal onto a wall of said buoy,        measuring a representative response echo resulting from rebound of the said ultrasound signal within said buoy,        comparing said representative response echo with said representative standard echo,        determining the presence or absence of water in the buoy depending on the results of said comparison.        
According to particular features, comparison of the representative response echo with the representative standard echo consists in calculating their difference in amplitude. According to particular features, determining the presence or absence of water in the buoy consists in comparing a calculated difference in amplitude with a predefined threshold, and                if said difference in amplitude does not exceed said threshold, concluding that the buoy does not contain water, or        if said difference in amplitude exceeds said threshold, concluding that the buoy does contain water.According to particular features, the threshold is 5 dB.        
Thanks to these provisions, one can eliminate potential uncertainties arising notably from noise and the conditions of under which the method according to the invention is implemented. According to particular features, the step of defining the representative standard echo notably comprises:                transmitting an ultrasound signal to a wall of a benchmark buoy filled with water, at a given location of said wall, then        measuring a first series of standard echoes, and        transmitting the same ultrasound signal to the wall of said benchmark buoy filled with air at the same location on said wall, then        measuring a second series of standard echoes, then        calculating the difference between each standard echo in the first series and each corresponding standard echo of the second series, then        selecting a pair of standard echoes having the largest difference,        selecting one of the two echoes of said pair of standard echoes.        
According to particular features, selection of the pair of standard echoes having the greatest difference is performed on pairs of standard echoes beyond the second one and below the twenty-first one. According to particular features, the step of defining a representative standard echo is performed using numerical simulation and/or testing on a representative model. According to particular features, the step of measuring representative response echo comprises notably:                measuring a series of response echoes, then        selecting the response echo corresponding to the selected representative standard echo.        
According to particular features, the transmission of ultrasound signals and the echo measurements are performed by means of a transducer.
The invention also provides apparatus for detecting water in a buoy for tensioning a riser of a marine installation, said apparatus comprising:                a human-machine interface        an ultrasound transducer applied to a wall of said buoy, said transducer transmitting an ultrasound signal and measuring a representative response echo caused by rebound of said signal within said buoy,        a data memory that stores a predetermined representative standard echo and a representative response echo measured by the transducer,                    a program memory comprising:            a program for transmitting the said ultrasound signal,            a program for measuring said response echo,            a program for comparing an amplitude of the representative response echo with the representative standard echo            a program for determining the presence or absence of water in the buoy dependent on the result of said comparison,                        a computer for controlling said human-machine interface, said transducer and said data and program memories.        
Other features and advantages of the invention will appear on reading the following description of a preferred embodiment of the invention, given by way of example and with reference to the accompanying drawings.